1. Field of the Invention
Broadly speaking, this invention relates to methods for measuring the parameters of a filament. More particularly, this invention relates to methods for measuring properties of clad optical fibers, the like.
2. Discussion of the Prior Art
In the manufacture of high quality optical fibers, for example, for use in optical communication systems, it is virtually mandatory that such important fiber parameters as core diameter and circularity, cladding thickness, and core and cladding refractive index be continuously monitored during the manufacturing process. Also, because an optical fiber is relatively fragile, it is important that the methods employed to measure these parameters do not damage the fiber in any way.
It is, of course, well known to employ a laser beam to measure the diameter of a fine metal wire. See, for example, Lasers in Industry, S. S. Charschan, editor, Van Nonstrand Reinhold Co. (1973) page 393 et seq. As taught in that publication, a laser beam directed at the wire to be measured generates the far-field Fraunhofer diffraction pattern of the wire. By measuring the spacing between successive maxima and minima in the diffraction pattern, and knowing the wavelength of the laser beam, it is a relatively easy matter to compute the diameter of the wire.
U.S. Pat. No. 3,709,610, which issued on Jan. 9, 1973 in the name of Herman A. Kreugle, suggests that this known technique may also be applied to measure the diameter of transparent, thermo-plastic filaments, such as rayon, nylon and acetate yarn. In a gross sense, this is true, bearing in mind that such fibers are not truly transparent but are more properly described as translucent. Thus, while the diffraction pattern generated from such a filament is complex, including contributions to the pattern caused by internal refraction through the yarn, the end result is essentially the same diffraction pattern that would be generated by an opaque filament, albeit of reduced contrast. Indeed, the Kreugle patent discloses several techniques for successfully detecting this reduced contrast diffraction pattern, including the technique of dying the yarn to render it opaque. See also the article by W. A. Farone and M. Kerker in the Journal of the Optical Society of America, Vol. 56 (1966) page 481 et seq., and the article by J. L. Lundberg in Journal of Colloid and Interface Science, Vol. 29, No. 3 (March 1969) at page 565 et seq.
Unfortunately, the measurement techniques disclosed by Kreugle are totally unsuited for use on high quality optical fiber Firstly, because these fibers are designed for use in low-loss optical communication systems, they are far more transparent than the translucent yarns measured by Kreugle. Thus, the contribution that the internally refracted rays make to the overall Fraunhofer pattern is considerably greater and cannot be ignored. In addition, reflection from the filament becomes increasingly significant and also cannot be ignored. Because of this, Kreugle's basic assumption, that the complex diffraction pattern generated by a translucent yarn can be treated as if it were an ordinary diffraction pattern, is incorrect when applied to the measurement of an optical fiber. Secondly, measurement of the diffraction pattern, even if it could be resolved, would not be accurate enough since the optical fiber is at least one order of magnitude smaller in diameter. Finally, and perhaps most important of all, an optical fiber typically comprises an inner core of a first refractive index and a thin outer cladding of a different refractive index. The measurement techniques disclosed by Kreugle, even if they could be applied to fiber optics, are incapable of measuring the thickness of the cladding layer and the core, or the relative refractive indices thereof, and at best, could merely measure the gross, overall diameter of the clad cable.
It is, however, known that a portion of a scattering pattern generated by a laser beam impinging on a transparent fiber can be used for measuring the diameter of the fiber. In this portion of the scattering pattern, interference between light reflected from the fiber and light refracted by the fiber causes fringes to appear. The distance between minima of the fringes is related to the diameter of the fiber. See "Interference Phenomena on Thin, Transparent Glass Filaments under Coherent Lighting," by Von Josef Gebhart and Siegfried Schmidt, Zeitschrift fur angewandte Physik, XIX. Band, Heft 2-1965.
The latter method, however, does not extend to clad fibers. It is desired to measure the diameter of the core and cladding thickness as well as the deviation for concentricity of the core of the clad fiber.